The existing capacitive touch apparatus, for example, applied to a typical portable terminal, comprises a data processing module and a touch screen including a touch panel and a display unit. A capacitive touch panel that allow multiple fingers to be detected generally includes driving electrodes arranged in parallel with one another and sensing electrodes arranged in parallel with one another across the driving electrodes in pairs. When the driving electrodes are driven, electric fields are formed between the driving electrodes and the sensing electrodes so that a mutual capacitor is formed at each intersection between one of the driving electrodes and its paired one of the sensing electrodes. The mutual capacitance includes intrinsic capacitance formed by that an electric field is free from external influence, and variable capacitance formed by that the electric field is subject to external influence. When the touch screen is touched, the finger or the special touch object changes the electric field to change the variable capacitance. The touch panel performs a scanning operation for a predetermined time interval period to collect touch events, and the touch panel converts a contact position into an electrical signal by sensing a change in capacitance formed by the electric field subject to external influence when a user's hand or object contacts the touch screen panel. The change of the mutual capacitance value coupling between X and Y axis electrode on the capacitive touch panel is sensed through the data processing module for judging the touched position of the touch panel to convert the contact position into the electrical signal. Accordingly, a baseline voltage which is the average voltage over an interval of time is measured at intersections in order to correlate touched X and Y electrodes into (X,Y) coordinate pairs when there is at least substantially no local electrostatic field generated by the touch of an instrument. When an instrument (e.g., finger) is applied proximal to the touch panel, a slight change in the output voltage magnitude is detected by the data processing module. The data processing module is controlled by at least one CPU (Central Processing Unit), such as computer, PDA (Personal Digital Assistant), various digital video players with display screens, etc.
As an example of conventional capacitive touch control, a coupling plate is provided and disposed to form a touch sensitive switch in WO-97/23738. In accordance with this example, a measurement cycle is performed that the coupling plate referred to as a key is charged using a drive circuit for a drive part of a measurement cycle and then this charge is measured by transferring the induced charge from the key to a charge measurement capacitor of a charge detection circuit. The charging and transferring parts of the cycle can vary widely and can be selected in accordance with the application concerned. Typically, a burst of measurement cycles are used to generate a plurality of signal sample values. The sensor can detect the presence of an object near the key as a result of a change in an amount of the charge induced onto the key, even in the presence of interfering substances.
Another form of touch sensitive control is disclosed in WO 00/44018. In this example a pair of electrodes is provided which act as a key so that the presence of a body such as a user's finger is detected as a result of a change in an amount of charge which is transferred between the two electrodes. With this arrangement, one of the electrodes (labeled X) is driven with a drive circuit and the other of the pair of electrodes (labeled Y) is connected to a charge measurement circuit which detects an amount of charge present on the Y plate when driven by the X plate. As disclosed in WO-00/44018 several pairs of electrodes can be arranged to form a matrix of sensing areas which can provide an efficient implementation of a touch sensitive two-dimensional position sensor. Such two dimensional capacitive transducing sensors are typically used with devices which include touch sensitive screens or touch sensitive keyboards/keypads which are used in, for example, consumer electronic devices and domestic appliances.
The induced electric fields between driving electrodes and sensing electrodes for a touch panel are vulnerable to be influenced by exogenous noises so that it can cause a disruption in accurately measuring a change in an amount of charge transferred due to touching the panel by an object (e.g., finger). The prior art touch sensing and measurement techniques have disclosed a signal subtraction method for reducing background noise impact as determining whether or not a touch has occurred. Firstly, prior to driving the touch pane, a signal A is acquired by measuring the potential of the sensing electrodes, and subsequently the driving electrodes is charged with a pre-determined current for a pre-determined time and then a signal B is acquired by measuring the potential of the sensing electrodes. Next, the signal subtraction method performs a subtraction operation to subtract signal B from signal A for acquiring a signal C. Finally, signal C is compared with a threshold value (e.g., a touch detection threshold), and signal C is determined to be a touch signal when signal C is above the threshold value. The charging and measurement procedures continue to be repeated as determining whether or not a touch has occurred.
A method of removing or at least reducing the effects of noise to improve the accuracy with which a signal measurement taken from a capacitively charged key is disclosed in U.S. Pat. No. 8,378,981. It provided a method and apparatus for sensing the presence of a body from a change in an amount of charge present on a capacitively charged key. The method includes performing a measurement burst which generates a plurality of signal sample values from a plurality of measurement cycles. Only those of the plurality of signal sample values, which are within the determined accepted range are used to detect whether the body is present by removing or at least reducing the effects of signal sample values, which are outside the determined accepted range of values. As a result, the effects of noise, such as square wave noise, which might otherwise cause the touch sensor to erroneously detect a body, can be substantially reduced.
The above-mentioned disclosures have provided methods for sensing the presence of a body from a change in an amount of charge present on a capacitively charged key and even improving the accuracy with which a signal measurement taken from a capacitively charged key by removing or at least reducing the effects of noise. However, the conventional measurement method may still make some mistakes to distinguish a touch signal as a result of fingers from a result of other conductive objects such as dirt, water or dust because the amount of voltage variation measured between them are not large difference. There is still a need of accurate recognition of user touch input while preventing from influence of noise signals.